1. Field of the Invention
This invention relates to a thick and controllably rigidified thermoplastic foam product and a method of manufacture thereof.
2. Description of Prior Art
Hot pin perforation bonding is used in the present invention. This bonding technique is shown in U.S. Pat. No. 4,758,297 - Calligarich, which discloses a method of using hot pin perforation bonding to laminate one ply of thermoplastic film to one ply of cellular fiber fabric. The process described therein is a continuous one using a heated roller with projecting pins.
Hot pin bonding has been used for many years in a commercial product by Ametek, Inc., the Assignee of the present invention, to produce a thin multi-layer flexible polypropylene foam sheet material.
In this case, the resultant product retains the flexibility of the original starting material sheets, and the pins generally do not fully perforate the material. Occasionally the hot pins may cause perforations in the sheet material. These perforations may be formed intentionally, to provide ventilation, or unintentionally due to overheated pins. Still, this product in all known forms is very flexible and relatively thin (less than one-half inch thick).
Thermoplastic (e.g. polyethylene and polypropylene) foam has been made by extruding or otherwise molding a sheet of resin, in which is incorporated a dispersed blowing agent. Upon decompression, as it leaves the extrusion die for example, the blowing agent expands and escapes as the resin solidifies. A foam product, ideally of flexible, micro-cellular structure, results. If the resin sheet is too thick, however, the blowing and hardening reactions are not coordinated and an unfoamed or poorly foamed product results.